The Space Interferometry Mission’s System Testbed-3 has recently integrated its Precision Support Structure and spacecraft
backpack (bus) on a pseudo-free-free 0.5 Hz passive isolation system. The Precision Support Structure holds a 3-baseline
stellar interferometer instrument. The architecture of the instrument is based on the current Space Interferometry Mission flight system design, and its primary purpose is to demonstrate nanometer class fringe stabilization using the path length feed forward technique. This paper describes the overall instrument architecture, brief theory of operation, and preliminary measurements.